AD6623BCSLASHPCB datasheet(37/40 Pages) AD

37 / 40 page

REV. 0

AD6623

–37–

Time Restriction CMEM Restriction

RCF

≤×









min

, 16

256

(31)

DMEM Restriction

where:

RCF

CIC

TSP

CLK

=×

(32)

De-interleaving the input data into multiple TSPs extends the

time restriction and may possibly extend the DMEM restriction,

but will not extend the CMEM restriction. Deinterleaving the

input stream to multiple TSPs divides the input sample rate to

each TSP by the number of TSPs used (NTSP). To keep the output

rate fixed, L must be increased by a factor of NCH, which extends

the time restriction. This increase in L may be achieved by increas-

ing any one or more of LRCF, LCIC5, or LCIC2 within their normal

limits. Achieving a larger L by increasing LRCF instead of LCIC5

or LCIC2 will relieves the DMEM restriction as well.

In a UMTS example, NTSP = 4, fCLK = 76.8 MHz, and fIN =

3.84 MHz, resulting in L = 80. Factoring L into LRCF = 10,

LCIC = 8, and LCIC2 = 1 results in a maximum NRCF = 40 due to

the time restriction. Figure 37 shows an example RCF impulse

response which has a frequency response as shown in Figure 38

from 0 Hz to 7.68 MHz (fIN

LRCF/NTSP). The composite RCF

and CIC frequency response is shown in Figure 38, on the same

frequency scale. This figure demonstrates a good approximation

to a root-raised-cosine with a roll-off factor of 0.22, a passband

ripple of 0.1 dB, and a stopband ripple better than –70 dB until

the lobe of the first image which peaks at –60 dB about 7.68 MHz

from the carrier center. This lobe could be reduced by shifting

more of the interpolation towards the RCF, but that would

sacrifice near in performance. As shown, the first image can be

easily rejected by an analog filter further up the signal path.

Scaling must be considered as normal with an interpolation

factor of L, to guarantee no overflow in the RCF, CIC, or NCOs.

The output level at the summation port should be calculated

using an interpolation factor of L/NTSP.

Programming Multiple TSPs

Configuring the TSPs for de-interleaved operation is straight

forward. All the Channel Registers and the CMEM of each TSP

are programmed identically, except the Start Hold-Off Counters

and NCO Phase Offset.

In order to separate the input timing to each TSP, the Hold-Off

Counters must be used to start each TSP successively in response

to a common Start SYNC. The Start SYNC may originate from

the SYNC pin or the MicroPort. Each subsequent TSP must have

a Hold-Off Counter value L/NTSP larger than its predecessor’s.

If the TSPs are located on cascaded AD6623s, the Hold-Off

Counters of the upstream device should be incremented by an

additional one.

In the UMTS example, L = 80 and NTSP = 4, so to respond as

quickly as possible to a Start SYNC, the Hold-Off Counter

values should be 1, 21, 41, and 61.

Driving Multiple TSP Serial Ports

When configured properly, the AD6623 will drive each SDFO

out of phase. Each new piece of data should be driven only into

the TSP that pulses its SDFO pin at that time.

In the UMTS example in Figure 35, L = 80 and NTSP = 4, so

each serial port need only accept every fourth input sample.

Each serial port is shifting at peak capacity, so sample 1, 2, and

3 begin shifting into Serial Ports B, C, and D before sample 0 is

completed into Serial Port A.

SDFOA

SDFOB

SDFOC

SDFOD

Figure 35. UMTS Example

RAM

COEF

FILTER

CIC

NCO

SUMMATION

BLOCK

CIC

NCO

CIC

NCO

CIC

NCO

DATA

RE-FORMATTER

DAC

RAM

COEF

FILTER

RAM

COEF

FILTER

RAM

COEF

FILTER

76.8 MSAMPLES/SEC

76.8MSPS

9.6MSPS

0.96

MCPS

0.96

MCPS

0.96

MCPS

0.96

MCPS

3.84 MCPS

COMLEX SIGNAL 32 BITS (16, I, 16 Q)

REAL OR IMAGINARY SIGNAL

Figure 36. Summation Block

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